The present invention relates to an improved corona air ionizer which eliminates microcontamination associated with conventional corona ionizers. Specifically, the invention provides for the elimination of ammonium nitrate buildup on the negative corona points and the elimination of bursts of submicron particles in corona ionizers by providing a stream of non-hydrogen-containing dry gas at the corona point during operation. Corona ionizers are commonly used in clean rooms, particularly clean rooms used in the manufacture of semiconductor devices.
Corona air ionizers have historically had a reputation for generating particulate contamination, while being very effective at reducing electrostatic charges on surfaces. Controlling electrostatic discharge (ESD) and reducing the sedimentation rate of small submicron aerosol particles are described in the article by K. Dillenbeck entitled "Selection of Air Ionization Within the Cleanroom" in Proceedings of the 32nd Annual Technical Meeting of the IES, pp 387-392 and in the article by R. P. Donovan et al entitled "The Dependence of Particle Deposition Velocity on Surface Potential" in 1987 Proceedings of the IES, pp 473-478. Unfortunately, corona air ionizers usually generate large quantities of small (less than 0.1 .mu.m) particles, primarily metal sputtered from the corona points themselves as noted in the article by B. Y. H. Liu et al entitled "Aerosol Charging and Neutralization and Electrostatic Discharge in Clean Rooms," in J. Envir. Sci, March/April 1987, pp 42-46 and in the article by M. Suzuki et al entitled "Effectiveness of Air Ionization Systems in Clean Rooms" in Proceedings of the 34th Annual Technical Meeting of the IES, pp 405-412.
Recently, an article by K. D. Murray et al entitled "Ozone and Small Particles Production by Steady State DC Hood Ionization: An Evaluation" in 1989 EOS/ESD Symposium Proceedings, pp. 18-22 and an article by K. D. Murray et al entitled "Hood Ionization in Semiconductor Wafer Processing: An Evaluation" in 1988 EOS/ESD Symposium Proceedings, pp 195-200, have shown that proper point design and material selection, in particular the use of plain tungsten corona points instead of thoriated tungsten corona points and careful control of the tip shape, can reduce the sputtered metal to insignificant levels, and have suggested that the major remaining source of contamination is ammonium nitrate (NH.sub.4 NO.sub.3) precipitated onto the negative corona points from the ambient air. Dispersive x-ray analysis ruled out tungsten as a major contributor to the remaining particles. Chemical analysis of the white precipitate on the negative points further showed it to be mostly NH.sub.4 NO.sub.3, making it plausible that the particles are also NH.sub.4 NO.sub.3. In addition to generating contamination, the precipitation necessitates replacing the points every month which contributes significantly to the cost of maintaining corona ionizers.
Experience has shown that the quantity of particles present often vary from none to tens of thousands of particles per cubic foot in a short time period. The highly intermittent character makes the emission episodes difficult to analyze. The problem is compounded of the lack of control over several relevant variables, such as humidity and temperature.